1. Field of the Invention
The present invention relates to a method of controlling the air/fuel ratio control system in an internal combustion engine.
2. Description Of Background Information
Various air/fuel ratio control systems for internal combustion engines are known from, for example, Japanese Patent Publication No. 55-3533, which systems regulate the air/fuel ratio of the mixture to be supplied to the engine toward a target air/fuel ratio by selecting values of control parameters for the air/fuel ratio control systems in response to the output signal of an oxygen concentration sensor disposed at the exhaust system of the engine thereby to regulate the volume of air or fuel to be supplied to the engine for the purification of the exhaust gases, the improvement of the fuel economy, etc. The air/fuel ratio control parameter may be, for example, a valve-open period in an intake side secondary air supply system, and fuel injection period in a fuel injection system.
It is, in this instance, to be noted that the oxygen concentration sensor used in the air/fuel ratio control systems does not become sufficiently active to produce desired output signals until the temperature of the sensor per se rises up to a certain level. It is therefore difficult to obtain accurate operation of the air/fuel control system during low temperature operation of the oxygen concentration sensor. It is, on the other hand, natural that the temperature of the oxygen concentration sensor is dependent upon the temperature of the exhaust gases since the oxygen concentration sensor is disposed within the exhaust gas flow. The temperature of the oxygen concentration sensor lowers during a low load operational condition, such as an idle condition, of the engine rather than high or medium load operational conditions since the temperature of the exhaust gases lowers during a light load operational condition. There is therefore a possibility that the oxygen concentration sensor will become inactive due to reduction of the temperature thereof below a certain level at a light load operational condition of the engine. During such an inactive condition of the oxygen concentration sensor, the sensor may produce an output signal representing a lean condition even though the actual air/fuel ratio is richer than the target air/fuel ratio, whereby the air/fuel ratio is regulated toward the rich side to cause an increase of unburned contents such as carbon monoxide and hydrocarbons in the exhaust gases.